Discriminator circuit



Filed April 30, 1954 AIWA..

,hlm @214.0105

ATTORNEYS United States Patent DISCRIMINATOR CIRCUIT Filed Apr. 30, 1954, Ser. No. 426,810

6 Claims. (Cl. 324-118) `The present invention comprises an improved method and means for discriminating between a desired sine wave signal of known phase and frequency and a band width of undesired sine wave signals of random phase and frequency. The invention is particularly adapted for use in increasing the apparent signal-to-noise ratio of maximum gain null detectors and will therefore be described with reference to such application.

The invention is predicated upon the fact that if a mixture of sine wave signals of random phase and frequency is sampled at regular intervals the statistical average of such samplings will be zero, that is, over a period of time, and at definite intervals, there will be as many measurements of one sign as there are of opposite sign and the sum of the magnitudes of such measurements will be substantially equal. Thus, if there is combined with such mixture of sine wave signals of random phase and frequency a desired sine wave signal of known phase and frequency, sampling of such combined signals at intervals corresponding to the peak of the desired sine wave signal will yield anoutput which when integrated is a measure solely of the peak value of `the desired signal. In maximum gain null detectors the desired signal is the unbalance signal of a bridge network or other measuring circuit. ln order `to detect when such signal approaches zero a high gain narrowqband ampliiier is ordinarily provided. However, t'he additionrof such amplifier and its input circuit introduces noise into the amplified output signal with the result that the desired signal is no longer pure but is combined with amixture of signals corresponding to the noise.

In accordance with the present invention the output from the amplifier, of a null high gain detector can be analyzed to detect and measure such part thereof as corresponds to the desired signal output from the bridge or other measuring circuit. The desired measurement is obof the output' signal.

For a better understanding of the invention and of specific embodiments thereof reference may be had tothe accompanying drawing of which:

Fig. l is a circuit diagram embodying the invention as appliedto a direct current null detector and employing an integrating circuit and a direct current vacuum tube voltmeter as the display means; and

Fig. 2 is a diagram of a `circuit embodying the invention as applied to an alternating current null detector and employing display means in which integration is performed by the eye.

2,942,188V Patented June 21, 1960 ri we In Fig. 1 a measuring circuit, the output of which yields the signalto be detected, is indicated symbolically asa direct' current bridge network 2 energized by a battery 4. An inverter 6 which is connected across the output terminals` of the bridge, converts the output signal therefrom yto, a square wave signal suitable for amplification by a high gain amplifier 8. The inverter 6 may be of any desired construction but is shown diagrammatically as comprising a chopper 10 the winding of which is energized from a source 12 of constant frequency vand the armature of which, under control of the winding, intermittently opens the output circuit of the bridge. The

inverter 6 could and ordinarily would, include knownY means for smoothing the output wave in order that the input to theV amplier 8 would be a sine wave rather than a square wave or alternatively, the amplifier 8 could include such smoothing means in its circuit.

The output from the amplifier 8 includes the amplified .signal from the bridge 2 and `also signals of random phase 26 being inserted between the source A12 and pulse-generator to time the occurrence of the pulses with the peaks of the desired sine 'wave signal. The pulses delivered to the winding of-the chopper 22 'are shown symbolically inFig. 1V adjacent the loop 28 encompassing the output leads of the pulse generator. The winding of chopper 22` when energized Iby a pulse closes a switch in line 16 between the amplifier 8 andan integrator 30. The output ofthe integrator 30 is delivered to a direct current vacuum tube voltmeter 32. The circuituof f,the integrator is indicated symbolically as comprising a series resistor 34 and shunt capacitors 36 and 38. y

With the above described circuit and with `suitable adjustrnentof the phaseof the input energy delivered to the pulse generator 24, the output of the amplifier 8 comprising the amplified desired signal from the bridge 2 and the mixture of signals corresponding to the generated noise, is .delivered to the `integrator 30 at intervals correspond-` ing to the peak ofthe desired signal.` The voltage impressed upon thevacuum tube voltmeter-32 is a steady voltage corresponding to the integration of the output of the sampler and will, as heretofere described, be an accurate measure of the peak of the desired signal. Thus,

"the circuit h-as a high -signal-to-noise ratio and measureplace .of the integrating circuit 30 `and voltmeter 32 and such an indicating tube when suitably pulsed could bridge network 4(l,is .diagrammatically indicated .as the A source olf` thesignal-l tombe measured and therefore.. no

`inverter to convert toalternatingwcurrent is required.

The output signal from the bridge 40 is delivered directly tothe. highgainnarrowband amplifier 31and `the-Aoutput thereof comprising the desired amplified signal mixed null indicator in place of the voltmeter of Fig. l. l For maximum signal-to-noise ratio the sampling pulse 'should be relatively narrow and should occur at peaks of the signal wave, that is at 90 or 270'a'ngles, once Y o t ce `withnoise as indicated by the loop 14, is delivered across the cathode and one defiecting electrode 43 of a 6AL7GT, schematically illustrated in the rectangle 42. I The other two defiecting electrodes of the tube are grounded. as isV the cathode of the 4tube and the output -lcad 1,8 from amplifien. The uorescent target of .the .tube is connected'to a source of positive potential, indicated as B-land the control grid of the :tube is con- 'nected through a capacitor 44 to the ungroundedoutput lead of the pulse generator 24. A suitable grid bias resistor 46 is connected between the cathode and grid ofthe tube. As in the circuit of Fig. l the pulse gen- 'erator is supplied with energy from the constant' frequency source l2 through the phase adjustor 26.

With the -above described circuit the target area 4responsive to the potential of deiiecting grid 43 will receive lelectrons from the cathode during each positive pulse applied to the control grid, that is, 'at intervals corresponding to the peaks of the desired signal. The -uovrescent pattern appearing on the target will thus correspond to the average potential of the deflecting grid 43 --when the desired signal is a maximum. Thus the tube performs the combined operations of sampling, integrating and indicating. Y

The invention has now been described with reference Y to two embodiments thereof. Although the Vspecific dis- Vplay means of Fig. l which Vcomprises a separate integrator and voltmeter, has been shownv in connection with a direct current null indicator, obviously such dis,-

play together with the sampler could be used inthe circuit of Fig. 2 in place of the Vindicator tube shown therein, and conversely, lthe combined `sampler and indicator tube of Fig. 2 could be employed in a direct current sampler, integrator'and each cycle. So far as the principle lof this invention is concerned, some improvement in signal-to-noise ratio will result so long as the pulses donot occur at V" or -180" angles of thesignal Wave and measurements could be made 'when frequency is an integralr multiple of -the number of pulses per second, that is when pulses occur once in two or more wavelengths.

VAs heretofore indicated, the bridge networks of Figs.

i2 and 4 of thel drawing Iare symbolic of lany measuring Vcircuit with which the invention is adapted for use. `No I attemptV has been made to illustrate specic .elements of generator energized from said source for creating a series of pulses of the frequency Aof the source and of short duration compared to the signal wave, phase adjusting means interposed between said ,source and saidpulse generator for causing coincidence of the created pulses with the Vpeaks ,of the amplified signal, means controlled 'by thecreated pulses for sampling the output of said amplifier and display means controlled by said sampling means @for signifying Ythe average of the samplings of the amplifier voutput to thereby discriminate betweenthev desired signal and `signals of random phase and frequency 'j in the amplifier output due `toy amplifier noise/,said display means comprising anintegrating kcircuitrand a volt- 'rneten said integrating circuit interconnecting said pulse Acontrolled means with said voltmeter.

u 2. `In anindicatingcircuit for detectngyanralternatng f 2,942,1ss

g current signal of known phase and frequency, the combination comprising a source of alternating voltage of the frequency of the signal to be detected, a high gain narrow band amplifier for amplifying the signal, a pulse generator energized from said source for creating a series of pulses of the frequency of the source and ofV short duration compared to the signal wave, phase adjusting means interposed between said source and said pulse generator for causing coincidence of thecreated pulses with the peaks of the amplified signal, 4means controlledV f amplifier 'output to thereby discriminate between the Yintegrated output of said integrating circuit. i

desired signal and signals of rando-m phase and frequency in the amplifier output due to amplifier noise, said display means comprising a fluorescent target of an indieating tube of the type including a cathode, control grid n and deflecting electrodes and said pulse controlled means comprising one of the deflecting electrodes` across' which and said cathode the amplifier output is impressed 'and the control grid across which and said cathode'the cre` ated pulses are impressed.

3. Means for increasing apparent signal-to-Vnoise ratio of a null detector circuit of the type including a high gain narrow band amplifier `for a desired signal of known phase and frequency which comprises a source of alternating voltage of the frequency of the signal, a pulse generator energized from. said source for creating a series of pulses of the frequency of the signal and of short duration compared to the signal wave, phase adjusting 4means interposed between said source and said generator for causing coincidence of the created pulses with fthe peaks of the amplified signal, a display devicefincluding integrating and indicating means, -and means controlled by the created pulses of intermittentlyfimpressing the amplier output on said display device to `yield an indication of the integrated input to the -dise Vsignal of the frequency vof the source, a high `gain nar- 45' i row band amplifier for the converted signal, a pulse generator energized from said source for generating aseries of pulses of the frequency ofthe source of short duration compared to the signal wave, phase adjusting means interposed between said source andsaid pulse Vgerierator for causing coincidence of the generated pulses with the peaks of the amplified converted signal, an integrating circuit, an electromagnetic chopper Venergized by the generated pulses and adapted when energizedto "impress the output of the amplifier'upon said Vintegrating `circuit and adirect currentvoltmeter forindicating y ,the

' 5.v The combination according lto claim 3 wherein said display means comprises an integrating circuit and L a Vacuum tube vvoltmeterY and said pulse controlledrne'ans comprises an electromagnetic chopper for intermitt ltly connecting said amplifier to said integrating circuit. 'f l 6. The combination according to claim Vwherein'said display means and said pulse controlled means Itogether comprise an indicating tube having a 4 target adapted to ;iuoresce when bombarded by electrons, a cathode for emission of electrons, a control grid .for Vcontrollingthe grid and cathode of said tube.'

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Schuchmann Oct. 18, 1938 Liston May 25, 1948 Williams Jan, 18, 1949 6 Labin et a1. Nov. 29, 1949 Yetter Jan. 6, 1953 Otis etal. May 18, 19,54

FOREIGN PATENTS Great Britain Nov. 10, 1944 

